Introduction

Klinefelter syndrome (sometimes called Klinefelter's, KS or XXY) is where boys and men are born with an extra X chromosome.

Chromosomes are packages of genes found in every cell in the body. Two types of chromosome, called the sex chromosomes, determine the genetic sex of a baby. These are named either X or Y.

Usually, a female baby has two X chromosomes (XX) and a male has one X and one Y (XY). But in Klinefelter syndrome, a boy is born with an extra copy of the X chromosome (XXY).

The X chromosome is not a "female" chromosome and is present in everyone. The presence of a Y chromosome denotes male sex.

Boys and men with Klinefelter syndrome are still genetically male, and often will not realise they have this extra chromosome, but occasionally it can cause problems that may require treatment.

Klinefelter syndrome is quite common, affecting around 1 in every 660 males.

This page covers:

Symptoms

Health issues

Causes

Testing and diagnosis

Treatments

More information and support

Content supplied by the NHS Website

Medically Reviewed by a doctor on 19 Jul 2016